Cutter for a dry-shaver driven by a motor

ABSTRACT

The invention relates to a cutter for a dry-shaver which is driven by motor, has a shear plate, in particular a flexible shear foil, provided with hair entry openings, and a cutter support which is adapted to be driven into rotation by a motor and carries at least one cutter, the cutting edge of the or each cutter cooperating with the shear plate, while the or each cutter is slightly tiltable with respect to the shear plate, the or each cutting edge being substantially straight and extending in a direction substantially at right angles to the direction of its movement with respect to the shear plate, whilst the cutters have, adjacent to the ends of their cutting edges, at least in as far as these ends cooperate with a shear plate part provided with hair entry openings, anti-damage portions which each have a curved outline with merges into the relevant cutting edge.

United States Patent [1 1 Tietjens Apr. 2, 1974 CUTTER FOR A DRY-SHAVER DRIVEN BY Primary Examiner-0thell M. Simpson A MOTOR Assistant ExaminerGary L. Smith [75] Inventor: Eduard Willem Tietjens, Drachten, Attorney Agent or Firm-Frank Tnfan Netherlands 73 A U S PhT [57] ABSTRACT Sslgnee I Corporanon New The invention relates to a cutter for a dry-shaver York, NY. I

which 18 driven by motor, has a shear plate, in particu- [22] Filed: June 5, 1972 lar a flexible shear foil, provided with hair entry open- Appl. No.: 259,914

Foreign Application Priority Data June 21, 1971 Netherlands 7108535 References Cited UNITED STATES PATENTS 10/1943 Rand 30/3465] X 11/1966 Ryan 30/436 X ings, and a cutter support which is adapted to be driven into rotation by a motor and carries at least one cutter, the cutting edge of the or each cutter cooperating with the shear plate, while the or each cutter is slightly tiltable with respect to the shear plate, the or each cutting edge being substantially straight and extending in a direction substantially at right angles to the direction of its movement with respect to the shear plate, whilst the cutters have, adjacent to the ends of their cutting edges, at least in as far as these ends c0- operate with a shear plate part provided with hair entry openings, anti-damage portions which each have a curved outline with merges into the relevant cutting edge.

4 Claims, 13 Drawing Figures PATENTEUAPR 21914 SHEET 1 [IF 7 PATENTEDAPR 2 I974 3.800.417

sum 2 [1F 7 v PAIENTEDAPR 21914 3800 417 SHEET 6 BF 7 Fig.11

Fig.13

CUTTER FOR A DRY-SHAVER DRIVEN BY A MOTOR BACKGROUND OF THE INVENTION Cutters of this type are known in which the centre of the radius of curvature of the curved outline of the antidamage portion lies on the cutter side remote from the shear plate, so that the outline has the form of a curve which initially forms a continuation of the cutting edge and gradually curves away from the shear plate. This gradualness of the curvature of the outline of the antidamage portion is of crucial importance for appropriate functioning, for the intention of the gradual curvature is to prevent a slight tilting movement of the cutter from causing a pointed end of the cutter to enter a hair entry opening, with consequent damage to the shear plate and the cutter and possibly even injury of the skin of the person using the dry-shaver, producing a burning feeling.

In cutters of this known type the said gradualness of the curvature of the outline of the anti-damage portions cannot be maintained in normal use of the dryshaver. Inevitable wear will remove the initial cutting edge after some time, so that the curved outline will make a sharp bend where it adjoins the new cutting edge produced by wear. Hence, in practice these anti-damage portions perform their desired function only as long as the dry-shaver is comparatively new, but after some time their effect will be increasingly unsatisfactory. The shape of the anti-damage portions adversely affects the cost of the cutter, an important factor in this respect being that the cutting edges and the adjoining curved outlines cannot be produced by surface grinding.

Several other attemps to obviate the aforementioned difficulties are known. For example, dry-shavers are known in which the cutters are thrust against the shear plate with a force such as to prevent the cutters from tilting under normal operating conditions. However, this solution gives rise to new difficulties, because owing to the comparatively large thurst forces required a considerable amount of friction will be produced between the cutters and the shear plates, which results in heating of the shear plates to a temperature disagreable to the user and in excessive wear of the shear plate and of the cutters. In addition, a comparatively powerful driving motor is required, with a consequent increase in size and cost of the shaver, whilst in cordless shavers the dry or rechargeable cells will be rapidly discharged.

Also, dry-shavers are known of a type in which the shear plate has a thickness and rigidly such that serious damage, such as fracture of the shear plate, is precluded. However, such a shaver cannot give a smooth shave, because this requires a shear plate having a thickness of at most 0.06 mm. Such thin shear plates, generally referred to as shear foils, are highly vulnerable.

Furthermore, dry-shavers are known in which the said problems are solved by causing the ends of the cutting edges of a cutter to cooperate with parts of the shear plate in which no hair entry openings have been formed. However, this reduces the effective shaving area of the shear plate. This reduction is not inconsiderable, because the areas of the shear plate which contain no hair entry openings must not be-too narrow in view of the manufacturing and assembling tolerances of the various component parts of the shaving head and the mobility of the cutters in the cutter support. This requires comparatively wide areas to ensure with sufficient certainty that the end of a cutting edge always is located in the appropriate area.

SUMMARY OF THE NEW INVENTION It is an object of the present invention to provide a cutter of the type mentioned at the beginning of this specification which is shaped so that the aforementioned difficulties do not arise, even after prolonged and intensive use of the shaver and even when using very thin shear foils in conjunction with cutters of the type described in our U.S. Pat. application Ser. No. 841,237, filed July 14, 1969, now U.S. Pat. No. 3,636,628 which are thrust to the foils with a very light pressure. The invention is characterized in that the curved outline of the anti-damage portion extends in the same plane in which the cutting edge is mainly movable, and curves away from the adjoining cutting edge into a direction substantially opposed to the direction of movement of the cutting edge relative to the shear plate.

This shaping of the anti-damage portions ensures that when as a result of a cutter tilting its end enters a hair entry opening no excessively large forces are exerted on the shear plate, because the anti-damage portion which, viewed in the direction of movement of the cutter, is rearwardly curved enables the cutter to gradually leave the opening. Wear of the cutter does not influence this procedure. In addition, the shape of the cutter does not become appreciably more complicated, so that the use of the invention hardly raises the cost of the shaving head.

To ensure satisfactory functioning of the anti-damage portions shaped in accordance with the invention there must be an appropriate relationship between their dimensions and the dimensions of the hair entry openings provided in the shear plate. The dimensions of the antidamage portions may be regarded as optimal if in comparison to the hair entry openings in the shear plates these portions have a size which is so large that damage of the aforementioned kind is excluded, but at the same time is as small as possible, because they contribute only in a limited degree to the cutting action of the cutters. Since the shape of the hair entry openings in the shear plates also influences these optimum dimensions, no dimensioning rules of general validity can be given. However, in the description of the accompanying F igures the theoretical background of the optimum dimensions of an anti-damage portion for a cutter which cooperates with a shear plate having circular hair entry openings will be discussed; from the theory developed general principles of dimensioning in other cases will be deduced.

An embodiment of the invention which is advantageous from the point of view of manufacturing technology is characterized in that the cutter is an integral piece of sheat material and the anti-damage portions are bent parts thereof.

Another embodiment of the invention is characterized that the anti-damage portions have the form of strips. This provides the advantage that the mass of the cutter need only slightly be increased. This is of particular importance for shaving heads of the type described in U.S. Pat. application Ser. No. 841,237, filed July 14, 1969, now U.S. Pat. No. 3,636,628 in which the cutters are spring loaded in a very slight degree only and must have small masses to enable then to follow the irregularities of the shear plate and its deformations during shaving. Obviously the strip-shaped parts must have a dimension at right angles to the shear plate such that even after prolonged use they will not be worn away entirely.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings;

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation and part sectional view of a shaving head which has cutters according to the invention and is mounted on the housing of a dry-shaver,

FIG. 2 is a plan view of the shaving head shown in FIG. 1, part of the shear plate being broken away to show the cutter support which is disposed beneath the shear plate and the cutters mounted in it,

FIG. 3 shows, on an enlarged scale, the lower part of a cutter which is not provided with anti-damage portions and is slightly tilted with respect to a shear plate so that one end of the cutting edge has entered a circular hair entry opening,

FIG. 4 shows the same situation as FIG. 3 in respect of a cutter which at both sides is provided with antidamage portions according to the invention,

FIG. 5 shows, on an enlarged scale, a cutter of the type shown in FIGS. 1 and 2 which cooperates a shear foil provided with hexagonal hair entry openings,

FIG. 6 is a plan view of a single round hair entry opening and part of a straight cutting edge of a cutter which cooperates with the edge of the opening, whilst furthermore lines, angles and points are shown which are of importance for illustrating the theoretical background of the invention,

FIG. 7 is a side elevation and part sectional view of the situation shown in FIG. 6,

FIG. 8 shows, likewise to illustrate the theoretical background of the invention, a perspective view of part of a shear plate and part of a circular hair entry opening, and furthermore lines, angles and points,

FIG. 9 is a perspective view similar to that of FIG. 8, in which, however, the forces are shown which act on the cutter at the point at which the cutter engages the boundary of a hair entry opening,

FIG. 10 is a plan view similar to that shown in FIG. 6, with the inclusion of a boundary line to the left of which the straight part of a cutting edge may cause damage,

FIG. 11 again shows the situation shown in FIG. 8 for the case in which the cutting edge of a cutter is at an obtuse angle 6 to the Y axis,

FIG. 12 again shows the plan view of FIG. 10, in which to the left of the said boundary line the optimally dimensioned curved outline of an anti-damage portion is shown in a form constructed according to a simplifled dimensioning rule, and

FIG. 13 illustrates how the cutting edges of 12 cutters according to the invention which still are interconnected by parts of the silver steel strip from which they have been punched, after hardening may simply be provided by grinding.

In the Figures corresponding elements are always designated by like reference numerals.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows the upper part of a dry-shaver 1 which comprises a shaving head 2 mounted on a housing part 3 which accommodates the motor. The shaving head 2 comprises a cylindrical wall 4 in the upper end of which a substantially flat shear foil 5 is mounted by means of a clamping piece 6 pressed into the cylinder. The cylinder 4 is screwed on an insertion piece 7 which is embedded in an envelope 8 made of synthetic material of the motor housing part 3. A cutter support in the form of a disc 10 is arranged beneath the shear foil 5 which is provided with a large number of hair entry openings 9. The disc 10 is drivably secured by means of a screw 11 to the shaft 12 of an electric driving motor which is accommodated in the motor housing part 3 but is not shown in the drawings. The motor shaft 12 is mounted in a sleeve bearing 13 pressed into the insertion piece 7. The disc 10 carries 12 cutters 14 which are slightly tiltable with respect to the shear foil 5 with which their cutting edges 15 cooperate. The cutting edges 15 of the cutters 14 are straight and extend in a direction substantially at right angles to the direction in which they are moved. Obviously the direction of movement of each cutter 14 mounted in the disc 10 is equal to the direction of rotation of this disc 10, which direction is indicated by an arrow P in FIG. 2. Each cutter 14 at either end of its cutting edge 15 has an anti-damage portion 16 which has a curved outline 1 7 which merges into the cutting edge 15; each cutter is mounted in a slit 18 in the disc 10. Each cutter 14 has a convex lateral edge 19 which presses against that wall of the slit 18 which is nearer the disc circumference, the cutter 14 otherwise having sufficient room in the slit 18 to enable it to be slightly tiltable with respect to the shear foil 5. Thin wirelike springs 20, which are suitably secured to the lower surface of the disc 10 and pass through openings 21 in the cutters 14, thrust the cutter again the lower surface of the shear foil 5 with a slight pressure.

According to the invention the curved outlines 17 of the anti-damage portions 16 extend in the same plane in which the cutting edge 15 is mainly movable. Obviously this plane, neglecting slight movements of the cutter 14 in a direction at right angles to the shear foil 5 and slight tilting movements of the cutters 14 relative to the shear foil 5 during shaving, is the plane containing the lower surface of the shear foil 5. The curved outline 17 bends away from the adjoining cutting edge 15 into a direction which is substantially opposite to the direction of movement P of the cutting edge 15 relative to the shear foil 5. In FIGS. 3, 4 and 5 the ends of the cutting edge 15 of the cutter 14 are designated by 22.

FIG. 3 shows, on a greatly enlarged scale, the lower part of a cutter 14 which is not provided with antidamage portions. The cutter is shown in a slightly tilted position relative to a shear foil 5 with which it cooperates and which is provided with circular hair entry openings 9. Owing to the tilted position of the cutter an end 22 of the cutting edge 15 may enter a hair entry opening. It will be seen from this Figure that, when the cutter 14 is driven in the direction of movement P, inevitably large forces will be exerted on the shear plate 5 and the cutter, because the end 22 which has entered a hair entry opening 9 will strike the wall of this openmg.

FIG. 4 again shows the situation of FIG. 3, but with the difference that the cutter 14 at both ends 22 of the cutting edge is provided with anti-damage portions 16 according to the invention. According to an embodiment of the invention the anti-damage portions 16 have the form of strips and hence have small masses. They are integral with the cutter 14 which is made of sheet material. Owing to the curved outline 17 of the anti-damage portion 16 the cutter 14, when it has assumed ,a slightly tilted position and continues to be driven inthe direction of movement P, may gradually move in a direction at right angles to the shear foil, so that the anti-damage portion 16 is enabled to move out of the hair entry opening 9 without exerting large forces on the surrounding parts of the shear foil 5.

FIG. 5 again shows on an enlarged scale a cutter 14 of the type shown in FIGS. 1 and 2 which cooperates with a shear foil 5 which is provided with hexagonal hair entry openings 9. Except for the anti-damage portions 16 cutters 14 of the type shown in FIG. 5 are described in U.S. Pat. application Ser. No. 841,237, filed July 14, 1969, now U.S. Pat. No. 3,636,628 and U.S. Pat. application Ser. No. 1 17,502 filed Feb. 22, 1971, now U.S. Pat. No. 3,740,843. The largest dimension of the hair entry openings 9 in the shear foil 5 is about 0.5 to 0.6 mm. The thickness of the sheet material of which the cutter 14 is made is only about 0.2 mm. In such cutters made of an extremely thin material it is of primary importance that the radius of curvature of the curved outline 17 of the anti-damage portion 16 should be suitably dimensioned, for it has been found that this radius of curvature must have a given minimum value to ensure satisfactory functioning of the anti-damage portion 16. In cases where the cutter 14 has a thickness greater than the aforementioned 0.2 mm considerations of material fracture may require the radius of curvature 17 to have a minimum value prescribed by the properties of the material, such as to ensure satisfactory functioning under any conditions. However, in cutters of small thickness, without the occurrence of restrictions due to the material, radii of curvature may be used which are smaller than the minimum value necessary for satisfactory functioning of the anti-damage portion.

Hereinafter the dimensions of an anti-damage portion 16 of a cutter which cooperates with a shear foil provided with circular hair entry openings will be discussed in detail. FIG. 6 is a plan view of a circular hair entry opening 9 formed in a flat shear foil 5. The Figure also shows part of a cutting edge 15 of a cutter which is not provided with an anti-damage portion. XOY coordinate axes the origin of which is in the centre of the hair entry opening 9 are shown in the plane in which the shear foil 5 and the cutting edge 15 cooperate. It is assumed that the cutting edge 15 moves parallel to the Y axis. The cutting edge 15 is in contact with the wall 22 of the hair entry opening 9 at a point A, and a line I, is the tangent to the circle 22 at the point A. The normal 1 through the origin to the tangent l, is at an angle a to the positive X axis.

FIG. 7 is a part side elevation, part sectional view of the situation shown in FIG. 6, the sectional plane containing the X axis and being at right angles to the XOY plane. FIG. 7 shows the tilted position of the cutter 14 the end 22 of the cutting edge 15 of which has just entered the hair entry opening 9. The angle at which the cutting edge 15 is tilted with respect to the XOY plane shown in FIG. 6 is indicated by an angle B between a line 1 which is the imaginary continuation of the cutting edge, and the upper surface of the shear foil. FIG. 7 shows a third coordinate axis which extends at right angles to the XOY plane and is referred to as the Z axis. To get of the hair entry opening 9 the cutter 14 must move not only in the Y direction but also in the Z direction, that is away from the shear foil.

FIG. 8 is a perspective view of the situation shown in FIGS. 6 and 7. Instead of the cutting edge 15 of the cutter 14 only the line 1 is shown. An arrow P,, indicates the instantaneous direction of movement of the line 1 and is at an angle 7 to the arrow P, which latter arrow lies in the XOY plane and is the projection of the arrow P,,,. The direction of movement of the cutting edge 15 and hence the line 1 lie in a plane which is defined by the line 1 and the tangent l, to the circle 22. As will be seen from FIG. 8:

tan y CD/BC; tan a AC/BC and tan B CD/AC from which follows:

tan 'y= tan a tan B.

The point B is the point in which the tangent 1 to the circle 22 intersects the Y axis, point C is the point in which the projection 1 of the line 1 on the XOY plane intersects the Y axis, and point D is the point in which the line 1 cuts the ZOY plane. From the expression for tan 3 it follows that the angle 7 increases with increase in a and B. The angle B, which is the tilting angle of a cutter relative to the shear foil, has a maximum value for a given dry-shaver construction. As FIG. 8 shows, the angle a has a value which varies from 0 to about Because of the predetermined relationship for a this means that 'y also varies between 0 and 90. If the angle y should actually become 90, the cutter would have to move perpendicularly, i.e., in the direction of the Z axis, to get out of the hair entry opening 9, while the direction of movement P of the cutter support 10 lies in the XOY plane. To prevent the shear foil 5 from being damaged ways must be found to prevent the occurrence of this situation.

In FIG. 9 a frictional force W acting on the cutter in the point A and the associated normal force N are shown which are produced under the influence of a driving force K which acts on the cutter in the direction of the arrow P. With a gradual increase of the angle 3 at a given instant the forces N, W and K which act in the point A will be in equilibrium. At this instant the angle (12 between a produced part of the arrow P and the normal force N satisfies the following relationship:

tan mu.r =f= where f is the coefficient of friction between the cutter and the shear foil 5. As will be seen from FIG. 9, we always have:

-EQFvhav wit a wtaa qw nsd m s it is desirable to ensure that:

From this follows:

tan a tan B tan (90 or tan a tan B l/tan 4),,

and hence tan a l/ftan B.

From the above relationship between the angle a, the coefficient of friction f and the angle B the maximum permissible angle a can be calculated for a given construction of a dry-shaver. When this angle is designated by a, it follows from FIG. 10, which is an elevation to that shown in FIG. 6, that the situation will be safe as long as the point of contact A between the cutting edge 15 of a cutter and the rim 22 of a circular hair entry opening 9 lies to the right of a boundary line I, shown as a broken line. It follows the Figure that the distance .r between the boundary line I,, and the Y axis satisfies the relationship:

where R is the radius of the hair entry opening 9. This means that the part of the cutting edge 15 which may become located to the left of the boundary line 1,, must have a shape such as to preclude damage.

To permit of a clear understanding of the invention it will also be necessary to ascertain the influence which an inclined position of a cutter with respect to its direction of movement has on the situation in which the forces K, N and W shown in FIG. 9 are balanced.

In FIG. II a line I is at an angle 8 to the Y axis. This angle is frequently referred to as the cutting angle. In addition to the points A, B, C and D FIG. 11 also shows a point E on the Y axis, which is the foot of a perpendicular l drawn from A to the Y axis. From FIG. 11 follows:

tan y DC/CB and tan ,3 DC/AC and furthermore the following relationships:

tan 180 6) =tan 8 AE/EC and tan a AE/BE.

From these relationships the following relationship may be deduced:

tan y (tan B tan a)/(sin 8 tan a cos 5) This formula shows thaty= 90if 01 8; this means that at this instant upward movement of the cutter is no longer possible. However, if care is taken to ensure that a always is greater than 6, such a situation will never occur. This is the fundamental idea on which the invention is based, because if affords the possibility of constructing an anti-damage portion.

Hereinbefore we have deduced the relationship:

7 E 900 bums By means ofthis relationship the preceding relationship may be written:

I/tan (b tan ,8 tan a/sin 8 tan a cos 8 which may may be rearranged to:

tan di tan ,8 sin 8 /tan a cos 8 Since the angles and B are known quantities, for every 8 an associated a can be calculated, permitting the theoretically correct form ofa cutter in the unsafe area", i.e., the area to the left of the boundary line 1,, in FIG. 10, to be determined.

For a practical case in which the angle [3 was found to have a maximum of and the maximum coefficient of friction was assumed to be 0.3 the theoretically optimum curved outline of the anti-damage portion was determined by means of the last of the aforementioned relationships. This theoretically optimum curved outline was found to approximate with a high degree of accuracy to the curved outline which is shown in FIG. 12 and has the form of an arc of a circle which starts from the point of intersection of the circular wall 22 ofthe hair entry opening 9 and the boundary line I and has a radius equal to R x. Practice has shown that such an arc ofa circle is highly satisfactory. It has further been found that owing to the choice of 0.3 for the value of the coefficient of friction, which value may be regarded as safe, the curved outline 17 need not extend as far as is shown in FIG. 12, but may be made shorter without impairing satisfactory functioning of the anti-damage portion. It has been found to be sufiicient for the curve 17 to be extended until the angle 8 which corresponds to an angle a I I540.

Obviously satisfactory functioning of the antidamage portion is always ensured when a curved outline 17 is used the radius of curvature of which is greater than the radius of curvature R .r shown in FIG. 12. Satisfactory operation is also ensured when the end 24 of the curved outline 17 is prolonged in the direction of the negative Y axis. Such deviations from the optimum dimensions do not affect satisfactory functioning of the anti-damage portion as such, but they adversely affect the mass of the cutter, i.e., they unnecessarily increase the mass of the cutter.

In a practical embodiment of the invention in a dryshaver having a shaving head as shown in FIGS. I and 2 very good results were obtained when using an extremely thin shear foil having a thickness of only about 40 ,uprovided with hair entry openings have diameters of about 0.55 mm, and cutters made of sheet material having a thickness of only 0.2 mm. In this embodiment the radius of curvature R x of the curved outline 17 had a value of 0.4 mm.

Obviously, such a radius of curvature may readily be achieved when the sheet material has a thickness of 0.2 mm. In cutters made of sheet material of greater thickness such a small radius of curvature may be impracticable in view of a fracture of the material, as has been stated hereinbefore. In such cutters the minimum radius of curvature which is achievable with regard to material considerations may be used and will automatically be satisfactory, because it will always be greater than the radius of curvature R x of FIG. 12.

Using the above outlined theory which is based on hair entry openings having a circular outline, cutters may be designed for hair entry openings of different shapes also, for it has been found that the most important factor is to ensure that the cutting angle 8 always satisfies the aforementioned requirements.

In a shaving head having a flat shear plate, as shown in FIG. 1, the cutting edge 15 and the curved outlines l7 lie in a flat plane. This enables the cutters to be provided with cutting edges by means ofa simple grinding operation.

FIG. 13 shows twelve cutters 14 which are still interconnected via connections 26 by a part 25 of the silver steel strip from which they have been punched. The assembly is clamped in a carriage 27 which is partly shown in section and is movable along a bed 29 of a grinding machine 30 in the direction indicated by an arrow 28. The machine further comprises a grinding wheel 32 adapted to rotate about a fixed axle 31 and driven by a motor, not shown. By moving the carriage 27 underneath the grinding wheel 32 the cutting edges 15 and the curved outlines 17 may rapidly and simply be provided.

What is claimed is:

1. In a dry-shaver including a housing, a rotary motor with a central axis, a cutter-support rotated by said motor, a shear plate with hair-openings therethrough carried by said housing and situated generally normal to said axis, the support including a plurality of slots, each extending axially therethrough and extending generally radially from said axis and defining a planar space, a plurality of cutters, one secured in each of said slots and moved in a circular path by rotation of said cutter support and having a top edge extending axially toward and contacting said shear plate, means urging each cutter toward said shear plate and permitting said cutter to tip about an axis generally normal to the plane of said slot, each cutters top edge defining a line, generally transverse to the cutters path of movement with ends of the edge bent about a smooth curve to a direction generally normal to said straight line to form antidamage portions extending generally along said path.

aid cutter.

2. Apparatus as defined in claim 1, wherein the cutter entirely consists of a single piece of sheet material, the anti-damage portions being bent parts of the sheet material.

3. Apparatus according to claim 1, wherein the antidamage portions are strip-shaped.

4. Apparatus according to claim 1 wherein said means urging each cutter toward the shear plate comprises a spring wire having one part secured in said cutter support and another part flexed and contacting g UNITED STATES PATENT OFFItE CERTIFICATE 0F QURRECTION Patent No. 3800417 Dated April 2, 1974 l egg EDUARD WILLEM TIETJENS It is certified that error appears in tha above-identified patent and that said LettersPatent are hereby carrected as shown below:

Col. 10, line 12, change "aid to -said-- Signed and sealed this 1st day of October .1974.

(SEAL) Attest:

c. MARSHALL DANN Commissioner of Patents MCCOY M." GIBSON JR. Attesting Officer 

1. In a dry-shaver including a housing, a rotary motor with a central axis, a cutter-support rotated by said motor, a shear plate with hair-openings therethrough carried by said housing and situated generally normal to said axis, the support including a plurality of slots, each extending axially therethrough and extending generally radially from said axis and defining a planar space, a plurality of cutters, one secured in each of said slots and moved in a circular path by rotation of said cutter support and having a top edge extending axially toward and contacting said shear plate, means urging each cutter toward said shear plate and permitting said cutter to tip about an axis generally normal to the plane of said slot, each cutter''s top edge defining a line, generally transverse to the cutter''s path of movement with ends of the edge bent about a smooth curve to a direction generally normal to said straight line to form anti-damage portions extending generally along said path.
 2. Apparatus as defined in claim 1, wherein the cutter entirely consists of a single piece of sheet material, the anti-damage portions being bent parts of the sheet material.
 3. Apparatus according to claim 1, wherein the anti-damage portions are strip-shaped.
 4. Apparatus according to claim 1 wherein said means urging each cutter toward the shear plate comprises a spring wire having one part secuted in said cutter support and another part flexed and contacting said cutter. 